Traditionally, valuable documents such as banknotes are printed on cotton-fiber paper substrates, which are inherently opaque. In order to combat counterfeiting and provide better durability, banknotes are now being developed with substrates that allow incorporation of complex security features. Banknote security has seen a paradigm shift with the advent of polymer substrates, which are optically transparent. When banknotes are printed on polymer substrates, an area of substrate is left free or transparent of any background and graphics so that an opaque material cannot be used for counterfeiting banknotes. The transparent area is hereinafter referred to as a “transparent window”. The transparent window may sometimes extend from one edge of the note to the other.
Typically, electronic transaction systems, such as vending machines, include currency handling units having one or more sensors to determine both authenticity and progress of the banknote along a transport path. The traditional sensors include a source of light that is generally placed along the transport path such that the angle of incidence of light is normal to the surface of the banknote. The ratio of the reflected light from the banknote to transmitted light through the banknote helps determine whether a banknote is present or not. However, banknotes with transparent windows may not be detected by the traditional sensors as light transmits almost completely through the banknote. As a result, a light detector detecting transmitted light energy sees it as an absence of bank note or a trailing edge/end of a banknote. This problem is particularly pronounced in cases where the transparent window extends across the width of the banknote. Inaccurate detection of transparent windows leads to miscalculation of length of the banknote, which then causes a valid banknote to be rejected as being too short. The miscalculation of length also causes the electronic transaction system to see two or more banknotes instead of one and the banknotes may be double counted causing problems in, for example, recycling type applications.